Intake valves of engines are positioned in an intake port disposed between the air intake and a combustion chamber. During an air intake stroke, a cam or rocker arm pushes the intake valve open and allows a fuel mixture to enter the combustion chamber. Further, exhaust valves are positioned in an exhaust port disposed between the combustion chamber and an exhaust flow passage. During an exhaust stroke, the cam or rocker arm pushes the exhaust valve open and combustion gases are expelled from the combustion chamber.
However, as engine power density increases and new combustion strategies are explored, engine exhaust valve operating temperatures requirements are increasing. These increasing temperatures are driven by both emissions regulations and industry-wide trends toward higher fuel efficiency and power density. The traditional limit has been about 725° C. operating temperature of continued exposure, with possible excursions up to 800° C. for a short period time. Standard valves are made of wrought iron-based austenitic stainless steel alloys, such as 23-8N (having a nominal composition of 23% Cr, 8% Ni, 2.5% Mn, 0.53 C, 0.43 N, balance Fe) or 21-4N (having a nominal composition of 21% Cr, 4% Ni, 9% Mn, 0.75% Si, 0.33 C, 0.30 N, balance Fe), however, these standard valves typically cannot operate above 750° C. for a sustainable period of time.
PCT Published Patent Application, WO/2004/079237 discloses a valve for an internal combustion engine, the method of its manufacturing and the heat-resisting titanium alloy containing the following relationship of components in mass %: aluminum 7,5-12,5, molybdenum 1,6-2,6, zirconium-1,4-2,4, silicon-0,1-0,2, yttrium-0,05-0,1, titanium-the rest are offered. The claimed alloy has α+α2+β-phase content with α2-phase based on the compound of Ti3Al dispersivily distributed in the α-phase. The claimed method consists in producing the valve from a cylindrical fillet by the deformation treatment with the preliminary heating and subsequent heat-treatment. The preliminary heating of the stem is conducted up to the temperature 5-20° C. lower than the temperature of complete polymorphic transformation (Tpc) of the alloy. The deformation treatment of the stem is conducted by wedge-transverse rolling. The deformation of the head is conducted by forging with the preliminary heating up to the temperature 5-50° C. higher than Tpc of the alloy, which corresponds to the temperature of the beginning of forging. The ending of forging is conducted at the temperature lower than Tpc, forming the disc-shape valve head and the smooth transition of the stem and the head. The technical result of the invention is obtaining the valve, providing the operation of the valve in a range of operating temperatures. However, valves made from this process are not able to be operated at temperatures in excess of 850° C. for an extended period of time.
If the valve is operated above its rated temperature, then surface instability may occur and oxidation layers may form on the surface and eventually flake off and be introduced into the combustion chamber. Additionally, the high temperature can cause the valve to fatigue or cause microstructure changes or its properties to degrade in the alloy causing a failure of the valve during use.
Thus, there is a need for an improved process that provides an engine valve that can operate at high temperatures for a sustainable period of time.